Evaporative heat exchange and air cleaning equipment is well known in the art and includes cooling towers, evaporative condensers, closed circuit evaporative fluid coolers, direct evaporative coolers, air washers and gas scrubbers, among others.
Evaporative heat exchange equipment is typically used to provide cooling to a process by rejecting heat from the process to the atmosphere. In operation, evaporative heat exchange equipment is used to contact heated water from a process with air. During this contact, heat and mass transfer occur simultaneously, resulting in a portion of the water being evaporated into the air. The energy required to evaporate the water is supplied from the sensible heat of the water which is not evaporated. Accordingly, the temperature of the non-evaporated water is reduced and cooling has been accomplished. The cooled water is then circulated back to the process wherein it picks up additional heat. The heated water then is recirculated back to the evaporative heat exchange equipment for cooling.
The water that is recirculated through evaporative cooling equipment becomes contaminated with impurities during operation. These contaminates are introduced to the recirculated water in several ways.
For example, airborne impurities become washed out of the air into the recirculated water with which it is brought in contact. These impurities are typically suspended in the recirculated water and over time, can possibly block or clog passageways within the system, or cause corrosion problems.
Also, dissolved solids contained in the water which is evaporated are left behind in the recirculating water during the evaporation process. Further worsening the situation is the fact that additional dissolved solids are introduced to the system via the make-up water which is added to the system to replenish the water which is evaporated. As a result of this evaporation and replenishment, the dissolved solids' level in the recirculated water can rapidly increase to unacceptable levels and can cause scaling of heat transfer surfaces and corrosion of system components.
Finally, biological organisms are constantly being added to the evaporative heat exchange system through the make-up water and from the air which is passed through the tower. The warm, moist, oxygen rich environment of evaporative cooling equipment represents a prime habitat for biological growth. Microbiological growth typically comprises algaes, slimes and bacteria. These growths can cause fouling of heat transfer surfaces resulting in reduced operating efficiency, and in severe cases, can completely block passageways within the system.
Due to the tendency of recirculated water to become contaminated during the operation of evaporative heat exchange equipment, it is typically necessary to treat the recirculated water to maintain its quality within acceptable levels. Generally, this treatment entails several different processes.
The level of dissolved solids present in the recirculated water is typically controlled through a process of bleeding off a portion of the high dissolved solids containing recirculating water and replacing it with fresh make-up water. In cases where simple bleed and fresh water replenishment is insufficient to control the scaling or corrosive tendency of the recirculating water, specialized scale and corrosion inhibiting water treatment chemicals are also used. Microbiological growth is generally difficult to effectively and efficiently control within evaporative cooling systems. This difficulty arises in part from the variable conditions which are present in such systems. This is especially true when evaporative heat exchange equipment is used in comfort cooling, or air conditioning systems where conditions under which the equipment operate are constantly changing.
For example, in an air conditioning system, air conditioning is typically required during the warm period of the day. During this time the load, or the amount of heat that must be rejected from the evaporative cooling equipment, is generally greatest. As the load increases, the temperature of the recirculated water increases and the amount of air that is passed through the tower may also increase. During such periods, the rate of microbiological growth within the system and the rate of addition of microbiological organisms to the system are at the highest levels.
Similarly, as the outdoor temperature begins to fall during the evening hours and the need for air conditioning decreases, the load on the evaporative cooling equipment and the temperature of the recirculating water also decreases. During such periods, the rate of microbiological growth and the rate of addition of microbiological organisms is lowered.
In general, biological growth within evaporative heat exchange systems is controlled through the addition of biocide chemicals to the recirculating water. Several different methods typically have been utilized to add biocides to the recirculating water of evaporative heat exchange equipment. A common approach is to pump liquid biocide chemicals from a drum or some other container directly into the recirculating water system. Generally, this is done on a timed basis with a pre-set amount of liquid biocide being added periodically, usually on an hourly or daily basis. In some cases, the amount of biocide chemical pumped into the system is done in response to a measured biocide concentration within the recirculating water system.
Unfortunately, such automatic biocide feed systems are expensive and require human attention to ensure that the proper amount of biocide chemical is being supplied to the recirculated water system. In addition, the operator must periodically replace the chemical drum or container when it is empty. Further, in those cases where biocide chemicals are added on a timed bases, biocide chemical is typically overfed during times when the rate of biological growth is low and underfed when the rate of biological growth is high.
Another method that has been used to supply biocide chemicals to the recirculated water of evaporative heat exchange equipment is to pass a side stream flow of the recirculating water through a bed of solid biocide chemicals. As the side stream flows through the bed of chemicals, a portion of the biocide chemicals are dissolved into the water stream. However, in order to prevent the particulate matter contained within the recirculating water from clogging or plugging the chemical bed, it is typically necessary to filter the side stream water passing through the chemical bed to remove such particulate matter prior to its entering the chemical bed.
A final method which has been used to add biocide and other treatment chemicals to recirculating water systems is to manually "hand-dose" or "slug-feed" the chemicals to the evaporative heat exchange equipment. Typically, this method is chosen on smaller systems, where the cost of expensive automatic feeding systems cannot be justified.
When using the "hand-dose" method, the operator must manually add large quantities of treatment chemicals to the recirculating water system on a periodic basis, usually once or twice weekly. Obviously, such a method results in erratic corrosion, scaling and microbiological control and can potentially cause serious health hazards when pathogenic organisms are present in the system. For example, if the time between slug doses of biocide is sufficiently large, and if the pathogen present in the system has a rapid growth rate, the population of the pathogen can increase between doses of biocide to potentially dangerous levels.
Another problem that must be addressed by any microbial control method is the potential for buildup of biofilms and deposits on system components. Most biocides are formulated and designed to attack microbiological organisms which are free in the bulk water. Such biocides are much less effective when used to control microorganisms contained within biofilms and deposits. This ineffectiveness could be due to the biocides' inability to penetrate the deposit or biofilm or due to the fact that the biocides are consumed by reactions in the water phase or at the surface of the film or deposit.
In light of the above, it is important in controlling these problems in evaporative heat exchange equipment that treatment chemicals are added in proportion to the need for chemical control within the system and that some level of treatment be maintained within the system at all times. Treatment chemicals should be added in proportion to their need within the system in order that the concentration of treatment chemical within a system can be held relatively constant. For example, this will result in the effective control of the microbial populations while using the minimum amount of biocide chemical. In addition, a continual presence of biocide, even at low concentrations, is needed to effectively control microbiological deposits and biofilms.